Devices for heating liquids in pipes or tubes are, e.g., described in the following patents: EP 82 025, GB-2 181 628, U.S. Pat. No. 4,156,127 and U.S. Pat. No. 4,038,519. An important disadvantage of such continuous flow heaters is their contamination by the precipitation of substances dissolved in the liquid. As is known, solubility is highly temperature-dependent. If the temperature of a solution in a pipe is increased, the solubility can be reduced. The dissolved substances are precipitated and are deposited on the pipe inner walls. This leads to pipe narrowing and, in the worst case, to pipe blockage. Thus, e.g., tap water used for coffee preparation contains more or less depositable fractions as a function of the geographical location and these fractions are hereinafter referred to as "scale". When tap water is heated from approximately 20.degree. C. or ambient temperature to approximately 95.degree. C. or boiling temperature, scale is precipitated from the liquid and deposited on the pipe inner walls. Pronounced scaling can be observed from about 60.degree. C.
The pipe scaling problem makes more difficult, or prevents in many cases, the use of continuous flow heaters for heating tap water. Pipes for such continuous flow heaters must be regularly and relatively frequently descaled or replaced, which would give rise to undesired interruptions to operation, as well as labor and material costs. Therefore, e.g., in conventional coffee machines, the water is heated with a solid electrical heating unit at the outlet from a water storage chamber. The hot water first flows through a riser into a boiling chamber, then through the coffee in the boiling chamber and finally through a filter into the coffee jug. For energy saving and time reasons, it is inappropriate to heat the electrical heating unit for preparing a single coffee serving. The solid electrical heating unit of conventional coffee machines has a high heat capacity and a relatively small heating surface, so that high thermal energy must be supplied to it in order to heat it and the heating of the solid unit and the water takes a long time, typically longer than 45 sec.